Biodegradable lubricating oil composition

ABSTRACT

The present invention provides a biodegradable lubricating oil composition containing 50% by mass or more of a synthetic ester base oil (A), 0.1 to 3% by mass of an amine-based antioxidant (B1), 0.1 to 3% by mass of a phenol-based antioxidant (B2), and 0.01 to 2% by mass of a sulfur-phosphorus-based extreme-pressure agent (C), wherein the transmittance at 3,005±1 cm −1  of a 0.1 mm-thick liquid film of the composition, as measured through IR absorptiometry, is 50% or more.

TECHNICAL FIELD

The present invention relates to a biodegradable lubricating oilcomposition containing a synthetic ester base oil.

BACKGROUND ART

Recently, from the aspect of environmental pollution control measures, abiodegradable lubricating oil has been being put into practical use alsoin the field of lubricating oil. A biodegradable lubricating oil isrequired to have a high biodegradation rate, and, therefore, use of alarge quantity of a mineral oil that is popularly used as a base oil inan ordinary lubricating oil is difficult. Consequently, the base oilmust be selected from a limited range of a natural vegetable oil, asynthetic polyalkylene glycol base oil, a synthetic ester base oil andthe like. Heretofore, among these, a synthetic ester base oil that isrelatively excellent in heat stability and oxidation stability is muchused.

As the synthetic ester base oil for use in a biodegradable lubricatingoil, there are known fatty acid diesters in which an aliphaticdicarboxylic acid is used as the carboxylic acid, hindered esters inwhich an aliphatic hindered polyol is used as the polyol, etc. Inaddition, for example, PTL 1 discloses a biodegradable lubricating oilprepared by blending a phenol-based antioxidant, a low base numbercalcium sulfonate and a triazole compound in a base oil containing ahindered ester in an amount of 50% by mass or more for enhancinglubrication performance, oxidation stability and anticorrosionperformance.

CITATION LIST Patent Literature

PTL 1: JP 2005-213451 A

SUMMARY OF INVENTION Technical Problem

Performance of lubricating oil compositions is being desired to improvemore year by year, and biodegradable lubricating oil compositions alsohave become required to have further prolonged lifetime and improvedwear resistance. However, for example, as described in PTL 1, eventhough a hindered ester is used as a base oil and different kinds ofadditives are blended therein, it is still difficult to sufficientimprove oxidation stability for lifetime prolongation, and in addition,wear resistance could not be improved sufficiently and the performancerequirements could not be satisfied.

The present invention has been made in consideration of theabove-mentioned problems and an object thereof is to provide abiodegradable lubricating oil composition having better wear resistanceand enhanced oxidation stability.

Solution to Problem

As a result of assiduous studies, the present inventors have found that,when a specific synthetic ester base oil is used as a base oil and whenthe transmittance at 3,005±1 cm⁻¹ of the lubricating oil composition ismade high, the resultant lubricating oil composition added with smallquantities of additives added thereto can have sufficiently improvedoxidation stability and wear resistance while maintaining goodbiodegradability, and have completed the present invention as describedbelow. Specifically, the present invention provides the following:

-   (1) A biodegradable lubricating oil composition containing 50% by    mass or more of a synthetic ester base oil (A), 0.1 to 3% by mass of    an amine-based antioxidant (B1), 0.1 to 3% by mass of a phenol-based    antioxidant (B2), and 0.01 to 2% by mass of a    sulfur-phosphorus-based extreme-pressure agent (C), wherein:

the transmittance at 3,005±1 cm⁻¹ of a 0.1 mm-thick liquid film of thecomposition, as measured through IR absorptiometry, is 50% or more.

-   (2) A method for producing a biodegradable lubricating oil    composition, including blending at least 0.1 to 3% by mass of an    amine-based antioxidant (B1), 0.1 to 3% by mass of a phenol-based    antioxidant (B2) and 0.01 to 2% by mass of a sulfur-phosphorus-based    extreme-pressure agent (C) in 50% by mass or more of a synthetic    ester base oil (A) to produce a biodegradable lubricating oil    composition wherein the transmittance at 3,005±1 cm⁻¹ of a 0.1    mm-thick liquid film of the composition, as measured through IR    absorptiometry, is 50% or more.

Advantageous Effects of Invention

According to the present invention, there can be provided abiodegradable lubricating oil composition having good wear resistanceand having enhanced oxidation stability.

DESCRIPTION OF EMBODIMENTS

Hereinafter the present invention is described with reference toembodiments thereof.

[Biodegradable Lubricating Oil Composition]

The biodegradable lubricating oil composition of one aspect of thepresent invention contains at least 50% by mass or more of a syntheticester base oil (A), as antioxidants (B), 0.1 to 3% by mass of anamine-based antioxidant (B1) and 0.1 to 3% by mass of a phenol-basedantioxidant (B2), and 0.01 to 2% by mass of a sulfur-phosphorus-basedextreme-pressure agent (C).

Further, in the biodegradable lubricating oil composition of one aspectof the present invention, the transmittance at 3,005±1 cm⁻¹ of a 0.1mm-thick liquid film of the composition, as measured through IRabsorptiometry, is 50% or more. The transmittance at 3,005±1 cm⁻¹ in IRabsorptiometry is an index of the amount of the unsaturated bonds in thebiodegradable lubricating oil composition, and when the transmittance is50% or more, the amount of the unsaturated bonds in the composition issmall.

In the lubricating oil composition containing a synthetic ester base oil(A) as the main ingredient (in an amount of 50% by mass or more), as inthis aspect, most of the unsaturated bonds are derived from thesynthetic ester base oil (A), and in this, by reducing the amount of theunsaturated bonds in the synthetic ester base oil (A), the transmittanceof the composition can be 50% or more. In this aspect, a synthetic esterbase oil (A) having a small amount of unsaturated bonds therein is usedto thereby reduce the amount of the unsaturated bonds in the lubricatingoil composition, and consequently, by adding small quantities of thespecific antioxidants (B) and the extreme-pressure agent (C), theoxidation stability and the wear resistance of the resultantbiodegradable lubricating oil composition can be sufficiently improved.

The transmittance of the lubricating oil composition is, from theviewpoint of more reducing the amount of the unsaturated bonds in thelubricating oil composition, preferably 55% or more, and for making thecomposition contain few unsaturated bonds, the transmittance is morepreferably 60% or more. The upper limit of the transmittance is 100%,but in view of the characteristics thereof, the transmittance isgenerally about 80% or less.

The components contained on the biodegradable lubricating oilcomposition are described in detail hereinunder.

[Synthetic Ester Base Oil (A)]

The synthetic ester base oil (A) may be adequately selected from esterbond-having synthetic base oils, and specifically may be selected from(A1) a polyol ester base oil being an ester of a polyol and an aliphaticmonocarboxylic acid, (A2) a diester base oil being an ester of analiphatic dicarboxylic acid and a monoalcohol, (A3) an ester base oilbeing a copolymer of an unsaturated dibasic acid ester and an α-olefin,etc. The synthetic ester base oil (A) may be one kind of an ester aloneor may also be a mixture of two or more kinds of esters.

The synthetic ester base oil (A) for use in the lubricating oilcomposition is, for reducing the amount of the unsaturated bonds thereinto thereby increase the transmittance of the lubricating oil compositionas above, preferably so selected that the transmittance at 3,005±1 cm⁻¹of a 0.1 mm-thick liquid film of the composition, as measured through IRabsorptiometry, could be 50% or more, more preferably 55% or more, andeven more preferably 60% or more so that the composition may contain fewunsaturated bonds. The upper limit of the transmittance of the syntheticester base oil (A) is 100%, but in view of the characteristics of thesynthetic ester base oil (A), the transmittance thereof is generallyabout 80% or less.

For example, in the case where two or more kinds of synthetic ester baseoils (A) are used in combination, preferably, two or more kinds ofthereof having a transmittance of 50% or more are mixed. One having atransmittance of 50% or more and another having a transmittance of lessthan 50% may be mixed and used, but in the case, the lubricating oilcomposition preferably contains a larger amount of the one having atransmittance of 50% or more than the another having a transmittance ofless than 50%.

The synthetic ester base oil (A) is contained in an amount of 50% bymass or more based on the total amount of the lubricating oilcomposition as above, but is preferably contained in an amount of 70% bymass or more, more preferably in an amount of 80% by mass or more, evenmore preferably in an amount of 90% by mass or more. The content of thesynthetic ester base oil (A) relative to the total amount of thelubricating oil composition is less than 99.8% by mass, but in orderthat the additives thereto to be mentioned below could be each in anadequate amount, the base oil content is preferably 99% by mass or less,more preferably 98% by mass or less.

The polyol ester base oil (A1) to be used as the synthetic ester baseoil (A) includes a hindered ester, that is, an ester of a hinderedpolyol having one or more of quaternary carbons in the molecule where 1to 4 methylol groups bond to at least one of the quaternary carbons, andan aliphatic monocarboxylic acid. More detailed examples of the hinderedpolyol include those having the following general formula (I):

wherein R¹ and R² each independently represent a hydrocarbon grouphaving 1 to 6 carbon atoms, or a methylol group, and n represents aninteger of 0 to 4.

In the above general formula (I), the hydrocarbon group having 1 to 6carbon atoms of R¹ and R² is preferably a linear chain or branched chainalkyl group, more preferably an alkyl group having 1 or 2 carbon atoms.n is preferably an integer of 0 to 2.

Examples of the hindered polyol represented by the general formula (I)include a hindered polyol such as a dialkylpropanediol (where the alkylgroup has 1 to 6 carbon atoms), a trimethylolalkane (where the alkanehas 2 to 7 carbon atoms), a pentaerythritol, etc., and a dehydratedcondensate thereof, and specifically include neopentyl glycol,2-ethyl-2-methyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol,trimethylolethane, trimethylolpropane, trimethylolbutane,trimethylolpentane, trimethylolhexane, trimethylolheptane,pentaerythritol, 2,2,6,6-tetramethyl-4-oxa-1,7-heptanediol,2,2,6,6,10,10-hexamethyl-4,8-dioxa-1,11-undecadiol,2,2,6,6,10,10,14,14-octamethyl-4,8,12-trioxa-1,15-pentadecadiol,2,6-di(hydroxyethyl)-2,6-dimethyl-4-oxa-1,7-heptanediol,2,6,10-tri(hydroxymethyl)-2,6,10-trimethyl-4,8-dioxa-1,11-undecadiol,2,6,10,14-tetra(hydroxymethyl)-2,6,10,14-tetramethyl-4,8,12-trioxa-1,15-pentadecadiol, di(pentaerythritol), tri(pentaerythritol),tetra(pentaerythritol), penta(pentaerythritol), etc.

Among these hindered polyols, trimethylolpropane, neopentyl glycol,pentaerythritol, and bimolecular or trimolecular dehydrated condensatesthereof are preferred; and above all, neopentyl glycol,trimethylolpropane and pentaerythritol are more preferred.

The aliphatic monocarboxylic acid to be used for the polyol ester baseoil (A1) includes a saturated aliphatic monocarboxylic acid having 5 to22 carbon atoms. The acyl group on the saturated aliphaticmonocarboxylic acid may be linear or branched. Examples of the saturatedaliphatic monocarboxylic acid of the type include a linear saturatedmonocarboxylic acid such as valeric acid, caproic acid, enanthic acid,caprylic acid, pelargonic acid, capric acid, undecanoic acid, lauricacid, tridecanoic acid, myristic acid, pentadecanoic acid, palmiticacid, heptadecanoic acid, stearic acid, nonadecanoic acid, arachic acid,behenic acid, etc.; a branched saturated monocarboxylic acid such asisomyristic acid, isopalmitic acid, isostearic acid,2,2-dimethylpropanoic acid, 2,2-dimethylbutanoic acid,2,2-dimethylpentanoic acid, 2,2-dimethyloctanoic acid,2-ethyl-2,3,3-trimethylbutanoic acid, 2,2,3,4-tetramethylpentanoic acid,2,5,5-trimethyl-2-t-butylhexanoic acid, 2,3,3-trimethyl-2-ethylbutanoicacid, 2,3-dimethyl-2-isopropylbutanoic acid, 2-ethylhexanoic acid,3,5,5-trimethylhexanoic acid, etc.

In esterification, one of these aliphatic monocarboxylic acids may beused singly or two or more kinds thereof may be used in combination. Thepolyol ester is generally a complete ester where all the hydroxyl groupsin a polyol are esterified, but within a range not having any negativeinfluence on the advantageous effects of the present invention, thepolyol ester for use herein may contain a small amount of an ester wherea part of hydroxyl groups are not esterified and remain as such.

As the diester base oil (A2), for example, an ester of a saturateddicarboxylic acid having 6 to 12 carbon atoms and an alkyl monoalcoholhaving 6 to 12 carbon atoms may be used. Examples of the saturateddicarboxylic acid include adipic acid, pimellic acid, suberic acid,azelaic acid, sebacic acid, undecane-diacid, dodecane-diacid, etc.Examples of the alkyl monoalcohol include a branched alkyl monoalcoholsuch as isooctanol, isononanol, isodecanol, 2-ethylhexanole, etc.; alinear alkyl monoalcohol such as n-octanol, n-nonanol, n-decanol,n-undecanol, n-dodecanol, etc. Preferred examples of the compoundsinclude dioctyl adipate, diisononyl adipate, diisodecyl adipate,di-2-ethylhexyl azelate, diisooctyl azelate, diisononyl azelate,di-2-ethylhexyl sebacate, diisooctyl sebacate, diisononyl sebacate,di-2-ethylhexyl dodecanedioate, etc.

The ester to be used as the diester base oil (A2) may be an ester of onekind of alkylmonoalcohol and a saturated dicarboxylic acid, or may alsobe an ester of two kinds of alkylmonoalcohol and a saturateddicarboxylic acid.

Specifically, the ester base oil (A3) is a copolymer prepared bycopolymerizing an ester of an unsaturated dibasic acid and amonoalcohol, and an α-olefin. The unsaturated dibasic acid to be usedhere includes maleic acid, fumaric acid, citraconic acid, mesaconicacid, itaconic acid, etc. The monoalcohol includes an alkyl monoalcoholhaving 1 to 20 carbon atoms. Among these, an alkyl monoalcohol having 3to 8 carbon atoms is more preferably used. The alkyl group of the alkylmonoalcohol may be linear or branched. Specifically, the alkylmonoalcohol includes methanol, ethanol, propanol, butanol, pentanol,hexanol, heptanol, octanol, nonanol, decanol, undecanol, etc.

The α-olefin is preferably one having 3 to 20 carbon atoms, morepreferably 6 to 18 carbon atoms. Examples of the α-olefin of the typeinclude propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene,etc.

Preferably, the ester base oil (A3) has a kinematic viscosity at 100° C.of 20 to 55 mm²/s, more preferably 25 to 45 mm²/s.

The above-mentioned various esters for use in the component (A) aregenerally produced by reacting a carboxylic acid and an alcohol, and, asa result, may have an ester structure formed of the above-mentionedcarboxylic acid residue and the alcohol residue. Accordingly, it is notnecessary to produce the component (A) by dehydration reaction of rawmaterials of the above-mentioned carboxylic acid and the alcohol, andthe component may be produced according to any other method using otherraw materials. For example, it may be produced according to antransesterification method.

Preferably, the synthetic ester base oil (A) contains the polyol esterbase oil (A1) as the main ingredient among the above-mentioned syntheticester base oils. Namely, the synthetic ester base oil (A) preferablycontains the polyol ester base oil (A1) in an amount of more than 50% bymass relative to the total amount of the synthetic ester base oil (A),more preferably in an amount of 70 to 100% by mass, even more preferably85 to 100% by mass.

Also preferably, the synthetic ester base oil (A) contains a polyolester base oil (A1-1) having a total carbon number of 23 to 50 in onemolecule as the main ingredient among the above-mentioned polyol esterbase oil (A1) for the reason of kinematic viscosity, etc. Namely, thesynthetic ester base oil (A) preferably contains a polyol ester base oil(A1-1) having a total carbon number of 23 to 50 in an amount of morethan 50% by mass relative to the total amount of the synthetic esterbase oil (A), more preferably in an amount of 70 to 100% by mass, evenmore preferably 75 to 100% by mass.

In the case where the synthetic ester base oil (A) contains a polyolester base oil (A1-1) having a total carbon number of 23 to 50 as themain ingredient as mentioned above, the base oil (A) may further containan ester base oil (A1-2) having a larger total carbon number in onemolecule than that of the component (A1-1) among the above-mentionedpolyol ester base oil (A1), and an ester base oil (A3) of theabove-mentioned copolymer as the side ingredients. Here, specifically,the ester base oil (A1-2) having a larger total carbon number in onemolecule than that of the component (A1-1) includes a polyol ester baseoil (A1-2) having a total carbon number of 51 to 80 in one molecule.

The synthetic ester base oil (A) preferably contains at least one kindof ester selected from the polyol ester base oil (A1-2) having a totalcarbon number of 51 to 80 in one molecule, and the ester base oil (A3)of the above-mentioned copolymer, in a ratio of less than 50% by massrelative to the total amount of the ester base oil (A), more preferablyin a ratio of 1 to 30% by mass, even more preferably 3 to 25% by mass.

Containing these (A1-2) and (A3) components as the side ingredients, thebiodegradable lubricating oil composition may be readily controlled tohave an adequate viscosity without losing oxidation stability and wearresistance.

For the polyol ester base oil (A1-1) having a total carbon number of 23to 50, one or more is adequately selected from the above-exemplifiedester base oil (A1), and preferred examples thereof include an ester ofneopentyl glycol (having 6 carbon atoms) with a saturated aliphaticmonocarboxylic acid having 9 to 22 carbon atoms, such as pelargonicacid, capric acid, undecanoic acid, lauryl acid, tridecanoic acid,myristic acid, pentadecanoic acid, palmitic acid, heptadecanoic acid,stearic acid, nonadecanoic acid, arachic acid, behenic acid, isomyristicacid, isopalmitic acid, isostearic acid, 2,2-dimethyloctanoic acid,2-ethyl-2,3,3-trimethylbutanoic acid, 2,2,3,4-tetramethylpentanoic acid,2,5,5-trimethyl-2-t-butylhexanoic acid, 2,3,3-trimethyl-2-ethylbutanoicacid, 2,3-dimethyl-2-isopropylbutanoic acid, 3,5,5-trimethylhexanoicacid, etc.; an ester of pentaerythritol (having 5 carbon atoms) with asaturated aliphatic monocarboxylic acid having 5 to 11 carbon atoms,such as valeric acid, caproic acid, enanthic acid, caprylic acid(octanoic acid), pelargonic acid, capric acid (decanoic acid),undecanoic acid, 2,2-dimethylpropanoic acid, 2,2-dimethylbutanoic acid,2,2-dimethylpentanoic acid, 2,2-dimethyloctanoic acid,2-ethyl-2,3,3-trimethylbutanoic acid, 2,2,3,4-tetramethylpentanoic acid,2,3,3-trimethyl-2-ethylbutanoic acid, 2,3-dimethyl-2-isopropylbutanoicacid, 2-ethylhexanoic acid, 3,5,5-trimethylhexanoic acid, etc.; and anester of trimethylolpropane (having 6 carbon atoms) with a saturatedaliphatic monocarboxylic acid having 6 to 14 carbon atoms, such ascaproic acid, enanthic acid, caprylic acid, pelargonic acid, capricacid, undecanoic acid, lauric acid, tridecanoic acid, myristic acid,2,2-dimethylbutanoic acid, 2,2-dimethylpentanoic acid,2,2-dimethyloctanoic acid, 2-ethyl-2,3,3-trimethylbutanoic acid,2,2,3,4-tetramethylpentanoic acid, 2,5,5-trimethyl-2-t-butylhexanoicacid, 2,3,3-trimethyl-2-ethylbutanoic acid,2,3-dimethyl-2-isopropylbutanoic acid, 2-ethylhexanoic acid,3,5,5-trimethylhexanoic acid, etc.

As the component (A1-1) among these, an ester of pentaerythritol ispreferred from the viewpoint of enhancing oxidation stability.

On the other hand, from the viewpoint of easiness in adequatelycontrolling the viscosity without using the above-mentioned component(A1-2) and the component (A3), the component (A1-1) is preferably anester of neopentyl glycol. The carboxylic acid in the ester of neopentylglycol is preferably a branched carboxylic acid, and more preferably asaturated aliphatic monocarboxylic acid having 16 to 20 carbon atoms.

Further, among the above-mentioned polyol ester base oil (A1-1), apolyol ester base oil having a total carbon number of 37 to 45 in onemolecule is preferred.

As the polyol ester base oil (A1-2) having a total carbon number of 51to 80, one or more may be selected from the polyol ester base oil (A1)that is an ester of a polyol and an aliphatic monocarboxylic acid asdescribed above, and preferred examples thereof include an ester ofpentaerythritol with a saturated higher aliphatic monocarboxylic acidhaving 12 to 18 carbon atoms, such as lauric acid, tridecanoic acid,myristic acid, pentadecanoic acid, palmitic acid, heptadecanoic acid,stearic acid, isomyristic acid, isopalmitic acid, isostearic acid,2,5,5-trimethyl-2-t-butylhexanoic acid, etc.; an ester oftrimethylolpropane with a saturated higher aliphatic monocarboxylic acidhaving 15 to 22 carbon atoms, such as pentadecanoic acid, palmitic acid,heptadecanoic acid, stearic acid, nonadecanoic acid, arachic acid,behenic acid, isopalmitic acid, isostearic acid, etc.

As the component (A1-2), use of an ester of trimethylolpropane amongthese is preferred. Further, using an ester of pentaerythritol as thecomponent (A1-1) is preferred along with using an ester oftrimethylolpropane as the component (A1-2). Using such a mixed ester canadequately control the viscosity characteristics of the lubricating oilcomposition without detracting from various characteristics of thecomposition.

The total carbon number in one molecule of the polyol ester base oil(A1-2) is preferably 51 to 70.

In order to make the synthetic polyester base oil (A) have atransmittance of 50% or more, the ester may be controlled to contain fewunsaturated bonds therein, and for example, as the component (A1), useof an ester of a specific polyol and a saturated aliphaticmonocarboxylic acid as mentioned above can attain the intended purpose.Here, many commercial products are available for a saturated aliphaticmonocarboxylic acid and an ester of the carboxylic acid, and suchcarboxylic acids or esters thereof may be adequately selected and usedhere. However, some commercial products of saturated aliphaticmonocarboxylic acids or esters thereof may contain unsaturated bonds,and the esters could not have a transmittance of 50% or more. This isbecause saturated aliphatic monocarboxylic acids are generally producedfrom animal oil and vegetable oil containing a large quantity ofunsaturated bonds.

On the other hand, the unsaturated bonds contained in animal oil andvegetable oil are generally hydrogenated and saturated during theproduction process, or are generally removed by purification.Consequently, in the component (A1) in this aspect, the saturatedaliphatic monocarboxylic acid to be used as the raw material ispreferably one having a high hydrogenation degree or one having a highpurification degree to have a small quantity of unsaturated bonds.

Similarly, also in the components (A2) and (A3), the raw materials ofalkyl monoalcohols and others are preferably ones having a highhydrogenation degree or having a high purification degree.

The base oil of the biodegradable lubricating oil composition may be theabove-mentioned synthetic ester base oil (A) alone, but may contain anyother base oil component than the above-mentioned synthetic ester baseoil (A) within a range not detracting from the advantageous effects ofthe present invention. Specifically, the base oil may contain at leastone selected from a polyether base oil such as a polyalkylene glycol, apolyvinyl ether, etc.; a mineral oil as exemplified by a paraffinicmineral oil, a napthenic mineral oil, an intermediate base mineral oil,etc.; a synthetic hydrocarbon oil such as a polybutene, a polypropylene,an olefin copolymer, etc. However, the content of the other base oilcomponent than the synthetic ester base oil (A) is preferably less than20% by mass based on the total amount of the lubricating oil compositionin order that the composition may secure high biodegradability asdescribed below, more preferably less than 10% by mass.

[Antioxidant (B)]

The biodegradable lubricating oil composition of this aspect contains,as antioxidants (B), both of an amine-based antioxidant (B1) and aphenol-based antioxidant (B2). In this aspect, these two antioxidantsare blended in the above-mentioned specific synthetic ester base oil(A), and therefore though the amount of each component to be blended issmall, the resultant composition can exhibit high oxidation stability.

The amine-based antioxidant (B1) includes a monoalkyldiphenylamine inwhich the alkyl group has 4 to 12 carbon atoms, such asmono-t-butyldiphenylamine, monooctyldiphenylamine,monononyldiphenylamine, etc.; a dialkyldiphenylamine in which the alkylgroup each has 4 to 12 carbon atoms, such as 4,4′-dibutyldiphenylamine,4,4′-dipentyldiphenylamine, 4,4′-dihexyldiphenylamine,4,4′-diheptyldiphenylamine, 4,4′-dioctyldiphenylamine,4,4′-dinonyldiphenylamine, 4-butyl-4′-octyldiphenylamine, etc.; apolyalkyldiphenylamine in which the alkyl group each has 1 to 10 carbonatoms, such as tetrabutyldiphenylamine, tetrahexyldiphenylamine,tetraoctyldiphenylamine, tetranonyldiphenylamine,di(2,4-diethylphenyl)amine, di(2-ethyl-4-nonylphenyl)amine, etc.; apolyalkyldiphenylamine which has 3 alkyl groups and in which the alkylgroup each has 1 to 10 carbon atoms, such as tetrabutyldiphenylamine,tetrahexyldiphenylamine, tetraoctyldiphenylamine,tetranonyldiphenylamine, di(2,4-diethylphenyl)amine,di(2-ethyl-4-nonylphenyl)amine, etc.; phenyl-α-naphthylamines asexemplified by an alkylphenyl-α-naphthylamine having at least one alkylgroup having 1 to 12 carbon atoms, such as methylphenyl-α-naphthylamine,ethylphenyl-α-naphthylamine, butylphenyl-α-naphthylamine,hexylphenyl-α-naphthylamine, heptylphenyl-α-naphthylamine,Octylphenyl-α-naphthylamine, nonylphenyl-α-naphthylamine,t-dodecylphenyl-α-naphthylamine, etc., or phenyl-α-naphthylamine, etc.

As the amine-based antioxidant (B1), using a dialkyldiphenylamine or analkylphenyl-α-naphthylamine among the above is preferred, and using adialkyldiphenylamine is more preferred.

<Phenol-Based Antioxidant (B2)>

The phenol-based antioxidant (B2) includes a monophenol-basedantioxidant and a bisphenol-based antioxidant.

The monophenol-based antioxidant includes an alkyl3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate (in which the alkyl groupincludes one having 4 to 20 carbon atoms, preferably 8 to 18 carbonatoms) such as n-octyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate,6-methylheptyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, n-octadecyl3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, etc.; a2,6-di-t-butyl-4-alkylphenol (in which the alkyl group has 1 to 4 carbonatoms) such as 2,6-di-t-butyl-4-methylphenol,2,6-di-t-butyl-4-ethylphenol, etc.; 2,4-dimethyl-6-t-butylphenol,2,6-di-t-amyl-p-cresol, etc.

The bisphenol antioxidant includes4,4′-methylenebis(2,6-di-t-butylphenol), 4,4′-bis(2,6-di-t-butylphenol),4,4′-bis(2-methyl-6-t-butylphenol),2,2′-methylenebis(4-ethyl-6-t-butylphenol),2,2′-methylenebis(4-methyl-6-t-butylphenol),4,4′-butylidenebis(3-methyl-6-t-butylphenol),4,4′-isopropylidenebis(2,6-di-t-butylphenol), 2,2′-methylenebis(4-methyl-6-nonylphenol), 2,2′-isobutylidenebis(4,6-dimethylphenol),2,2′-methylenebis (4-methyl-6-cyclohexylphenol),4,4′-thiobis(2-methyl-6-t-butylphenol),4,4′-thiobis(3-methyl-6-t-butylphenol),2,2′-thiobis(4-methyl-6-t-butylphenol),bis(3-methyl-4-hydroxy-5-t-butylbenzyl) sulfide,bis(3,5-di-t-butyl-4-hydroxybenzyl) sulfide,thiodiethylenebis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], etc.

The phenol-based antioxidant is preferably a monophenol-basedantioxidant among the above, and above all, an alkyl3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate is more preferred.

The biodegradable lubricating oil composition contains the amine-basedantioxidant (B1) in an amount of 0.1 to 3% by mass and the phenol-basedantioxidant (B2) in an amount of 0.1 to 3% by mass based on the totalamount of the composition, as mentioned above. The content of theseantioxidants (B1) and (B2) is controlled to be each 0.1% by mass or moreso that the biodegradable lubricating oil composition can be given highoxidation stability. In addition, the content thereof is controlled tobe each 3% by mass or less so that the biodegradable lubricating oilcomposition can exhibit the advantageous effects commensurate with thecontent and reduction in the biodegradability of the composition owingto the antioxidants (B) therein can be prevented. In particular, theamine-based antioxidant (B1) is often a factor of reducing thebiodegradability of the lubricating oil composition, but in this aspect,the antioxidant is used along with the above-mentioned specificsynthetic ester base oil (A), and therefore even a small amount of theamine-based antioxidant (B1) can sufficiently enhance the oxidationstability of the composition. Consequently, in this aspect, thereduction in the biodegradability can be minimized.

In addition, for more enhancing oxidation stability while preventingreduction in biodegradability, the content of the amine-basedantioxidant (B1) is preferably 0.2 to 2.5% by mass, more preferably 0.3to 1.8% by mass. From the same viewpoint, the content of thephenol-based antioxidant (B2) is preferably 0.2 to 2.5% by mass, morepreferably 0.3 to 1.5% by mass.

[Sulfur-Phosphorus-Based Extreme-Pressure Agent (C)]

The biodegradable lubricating oil composition of this aspect furthercontains a sulfur-phosphorus-based extreme-pressure agent (C). When thecomposition contains the synthetic ester base oil (A) as above, thecomposition could not exhibit extreme-pressure performance even thoughan extreme-pressure agent is added thereto. However, amongextreme-pressure agents, a sulfur-phosphorus-based extreme-pressureagent (C) is added thereto, and therefore the lubricating oilcomposition can sufficiently exhibit extreme-pressure performance andcan better wear resistance thereof.

The sulfur-phosphorus-based extreme-pressure agent (C) to be usedincludes monothiophosphates, dithiophosphates, trithiophosphates,monothiophosphate amine salts, dithiophosphate amine salts,monothiophosphites, dithiophosphites, trithiophosphites, etc. Amongthese, dithiophosphates are preferred.

From the viewpoint of bettering wear resistance, dithiophosphates havinga terminal carboxyl group are preferred among dithiophosphates. Whenhaving a terminal carboxyl group, the sulfur-phosphorus-basedextreme-pressure agent (C) can have an increased polarity, and thereforein this aspect using the above-mentioned specific synthetic ester baseoil (A) as the base oil, the sulfur-phosphorus-based extreme-pressureagent (C) can readily exhibit the function of an extreme-pressure agent.

Specific examples of the dithiophosphate having a terminal carboxylgroup include compounds represented by the following general formula(II):

wherein R³ represents a linear or branched alkylene group having 1 to 8carbon atoms, and R⁴ and R⁵ each independently represent a hydrocarbongroup having 3 to 20 carbon atoms.

In the formula (II), R³ is, from the viewpoint of bettering solubilityin base oil, preferably a linear or branched alkylene group having 1 to8 carbon atoms, more preferably a linear or branched alkylene grouphaving 2 to 4 carbon atoms, and even more preferably a branched alkylenegroup. Specifically, —CH₂CH₂—, —CH₂CH(CH₃)—, —CH₂CH(CH₂CH₃)—,CH₂CH(CH₃)CH₂— and —CH₂CH(CH₂CH₂CH₃)— are preferred; —CH₂CH(CH₃)— and—CH₂CH(CH₃)CH₂— are more preferred; and —CH₂CH(CH₃)— is even morepreferred.

R⁴ and R⁵ each are, from the viewpoint of bettering extreme-pressureperformance and bettering solubility in base oil, preferably a linear orbranched alkyl group having 3 to 8 carbon atoms, more preferably alinear or branched alkyl group having 4 to 6 carbon atoms. Specifically,the group is preferably selected from the group consisting propyl,isopropyl, butyl, isobutyl, t-butyl, pentyl, isopentyl, hexyl,2-ethylbutyl, 1-methylpentyl, 1,3-dimethylbutyl and 2-ethylhexyl groups.Among these, isobutyl and t-butyl are more preferred.

The biodegradable lubricating oil composition contains thesulfur-phosphorus-based extreme-pressure agent (C) in an amount of 0.01to 2% by mass based on the total amount of the composition, as mentionedabove. When the content of the sulfur-phosphorus-based extreme-pressureagent (C) is 0.01% by mass or more, the lubricating oil composition canbe given extreme-pressure property to better wear resistance thereof.When the content is 2% by mass or less, the composition can exhibit theeffect commensurate with the content to thereby prevent thebiodegradability and the oxidation stability of the biodegradablelubricating oil composition from being lowered owing to thesulfur-phosphorus-based extreme-pressure agent (C).

For more preventing biodegradability and oxidation stability from beinglowered and for more enhancing wear resistance, the content of thesulfur-phosphorus-based extreme-pressure additive (C) is preferably 0.02to 1% by mass, more preferably 0.03 to 0.5% by mass.

[Viscosity Index Improver]

The biodegradable lubricating oil composition of this aspect may containa viscosity index improver.

The viscosity index improver includes a polymethacrylate, a dispersivepolymethacrylate, an olefin copolymer (for example, anethylene-propylene copolymer, etc.), a dispersive olefin copolymer, astyrene copolymer (for example, a styrene-diene copolymer, astyrene-isoprene copolymer, etc.), etc. Among these, a polymethacrylateis preferred. The polymethacrylate usable as a viscosity index improvergenerally has a weight-average molecular weight of 10,000 to 70,000,preferably 20,000 to 55,000. The weight-average molecular weight is avalue measured through gel permeation chromatography and derived from acalibration curve drawn using polystyrene.

The content of the viscosity index improver is preferably 0.1 to 10% bymass based on the total amount of the lubricating oil composition, morepreferably 0.5 to 5% by mass.

[Triazole Compound]

The biodegradable lubricating oil composition of this aspect may furthercontain a triazole compound. The triazole compound acts as a metalinactivator, and imparts an anticorrosive effect against non-ferrousmetals to the biodegradable lubricating oil composition. Specificexamples of the triazole compound include benzotriazole,carboxybenzotriazole, 3-aminotriazole, 4-aminotriazole,2,5-diaminotriazole, 3-mercaptotriazole, and N-dialkyl (with 3 to carbonatoms)aminomethyl-1,2,3-benzotriazole such asN-diethylaminomethyl-1,2,3-benzotriazole, etc. Those having abenzotriazole skeleton (benzotriazole compounds) are preferred.

The content of the triazole compound is preferably 0.01 to 1% by massbased on the total amount of the lubricating oil composition, morepreferably 0.02 to 0.5% by mass.

[Rust Inhibitor]

The biodegradable lubricating oil composition may contain at least oneselected from an alkaline earth metal sulfonate and a succinate, as arust inhibitor. Containing a rust inhibitor, the biodegradablelubricating oil composition can have an increased corrosion-resistanteffect against metals such as iron, etc.

The alkaline earth metal sulfonate is one prepared by sulfonating analkylaromatic compound followed by converting it into an alkaline earthmetal salt thereof, and includes a calcium sulfonate, a magnesiumsulfonate and a barium sulfonate. Among these, a calcium sulfonate ispreferred. The alkaline earth metal sulfonate preferably has a lowbasicity, and specifically the total base number (TBN) thereof ispreferably 0 to 100 mgKOH/g, more preferably 0 to 50 mgKOH/g. The totalbase number is measured according to a perchloric acid method of JISK-2501. Using an alkaline earth metal sulfonate, the composition canadditionally exhibit a detergent-dispersant effect.

The alkenyl succinate includes a half ester of an alkenylsuccinic acidwith an alcohol such as a polyalcohol, etc.

One of the rust inhibitors may be used singly or two or more kindsthereof may be used in combination. The content of the rust inhibitor ispreferably within a range of 0.01 to 1.0% by mass based on the totalamount of the lubricating oil composition, more preferably 0.03 to 0.5%by mass.

(Other Additives)

The biodegradable lubricating oil composition may contain any otherextreme-pressure additive than the sulfur-phosphorus-basedextreme-pressure agent (C). Specifically, the other extreme-pressureagent includes a phosphorus-based extreme-pressure agent such as aphosphate, e.g., tricresyl phosphate (TCP), an acidic phosphate aminesalt, a phosphite, etc. The content of the phosphorus-basedextreme-pressure agent is preferably 0.1 to 2% by mass based on thetotal amount of the lubricating oil composition, more preferably 0.2 to1.5% by mass.

The biodegradable lubricating oil composition may contain any otheradditive than the above, such as an ashless dispersant, a pour pointdepressant, an anti-foam agent, a surfactant, a demulsifier, etc.

Examples of the ashless dispersant include a succinimide, aboron-containing succinimide, a benzylamine, a boron-containingbenzylamine, etc.

The pour point depressant includes an ethylene-vinyl acetate copolymer,a condensate of a chloroparaffin and a naphthalene, a condensate of achloroparaffin and a phenol, a polymethacrylate, a polyalkylstyrene,etc. The anti-foam agent may be a silicone anti-foam agent or anon-silicone anti-foam agent.

[Property of Biodegradable Lubricating Oil Composition]

The biodegradable lubricating oil composition of this aspect preferablyhas a biodegradation rate of 60% or more as measured in a degradationtest for chemical substances with microbes according to the 301B test ofthe OECD Test Guideline, more preferably 70% or more. In this aspect,the specific synthetic ester base oil (A) is used as the main component,and the amount of various additives of the antioxidants (B1) and (B2)and the sulfur-phosphorus-based extreme-pressure agent (C) is controlledto be a predetermined amount or less, and the biodegradation rate of thecomposition can be thereby increased.

The kinematic viscosity at 40° C. of the biodegradable lubricating oilcomposition is preferably 10 to 150 mm²/s, more preferably 15 to 100mm²/s. The viscosity index of the composition is preferably 130 or more,more preferably 135 or more. Having a kinematic viscosity and aviscosity index each falling within the range, the biodegradablelubricating oil composition can be adequately used as a lubricating oilin various uses to be mentioned hereinunder.

The biodegradable lubricating oil composition of this aspect can befavorably used, for example, for a hydraulic fluid that is a powertransmission fluid for use for power transmission, power control, bufferor the like in a hydraulic system; a lubricating oil or a universal oilfor transmissions of agricultural tractors, or construction or civilengineering machines; an oil for chain saws; a 2-cycle engine oil; anindustrial gear oil for wind-power generation, etc. Among these, thecomposition is more preferably used as a hydraulic fluid.

[Production Method for Biodegradable Lubricating Oil Composition]

A production method for the biodegradable lubricating oil composition inthis aspect includes blending at least 0.1 to 3% by mass of anamine-based antioxidant (B1), 0.1 to 3% by mass of a phenol-basedantioxidant (B2) and 0.01 to 2% by mass of a sulfur-phosphorus-basedextreme-pressure agent (C) in 50% by mass or more of a synthetic esterbase oil (A) to produce a biodegradable lubricating oil compositionwherein the transmittance at 3,005±1 cm⁻¹ of a 0.1 mm-thick liquid filmof the composition, as measured through IR absorptiometry, is 50% ormore. As described above, any other components than these components(A), (B1), (B2) and (C) may be blended in the biodegradable lubricatingoil composition.

The details of the components (A), (B1), (B2) and (C), and the othercomponents than these and details of the obtained biodegradablelubricating oil composition are as described above, and are thereforeomitted herein.

EXAMPLES

The present invention is described more specifically with reference toExamples, but the present invention is not whatsoever restricted bythese Examples.

Various properties of the lubricating oil composition were measured andevaluated according to the methods mentioned below.

(1) Kinematic Viscosity (40° C., 100° C.)

Measured according to JIS K 2283.

(2) Viscosity Index

Measured according to JIS K 2283.

(3) Acid Value

Measured in an indicator method according to JIS K 2501.

(4) IR Spectrometry

Using an IR spectrometer (trade name, FT-IR6200, manufactured by JASCOCorporation), a lubricating oil composition was introduced betweenpotassium bromide cells via a 0.1 mm-thick spacer to form a 0.1 mm-thickliquid film therebetween, and the transmittance thereof at 4,000 to 400cm⁻¹ was measured at a resolution of 4 cm⁻¹ for a number of 16 scans,and then the transmittance at 3,005±1 cm⁻¹ was read to be thetransmittance of the lubricating oil composition.

In place of the lubricating oil composition, a synthetic ester base oilwas introduced between the cells, and the transmittance of the syntheticester base oil was measured according to the same method as above.

(5) Shell Wear Test

Using a shell wear tester and according to ASTM D 2783, the load bearingperformance of the lubricating oil composition was evaluated under thetest conditions of a load of 294 N, a rotating speed of 1,200 rpm, atemperature of 50° C., and for a test period of 30 minutes. The resultwas expressed as the wear track (mm) by the test steel ball.

In this test, when the wear track is 0.5 mm or less, the wear resistanceis evaluated as good “A”, but when the wear track is more than 0.5 mm,the wear resistance is evaluated as insufficient “B”.

(6) RBOT Test

According to the rotating cylinder-type oxidation stability test of JISK 2514-3, the lubricating oil composition was tested at a testtemperature of 150° C. and under a pressure of 620 kPa, and the timetaken until the pressure lowered by 175 kPa from the maximum pressurewas measured.

In this test, when the RBOT value is 250 minutes or more, the testedcomposition is evaluated as good “A” since its oxidation stability issufficient in use, for example, as a compression hydraulic oil, but whenthe value is less than 250 minutes, the tested composition is evaluatedas not good “B” since the oxidation stability thereof is insufficient.

(7) ISOT Test

According to JIS K 2514-1, a copper/iron catalyst was made to exist in asample oil, and the sample oil was aged at a test temperature of 130° C.for a test period of 168 hours. A value calculated by dividing thekinematic viscosity at 40° C. of the aged oil by the kinematic viscosityat 40° C. of the unaged oil was referred to as a viscosity ratio. Inaddition, the acid value of the aged oil was subtracted from the acidvalue of the unaged oil to give an acid value increase.

Examples 1 to 4, Comparative Examples 1 to 6

Biodegradable lubricating oil compositions were prepared in the blendingformulation shown in Table 1, and tested to determine and evaluate theproperties thereof. The results are shown in Table 1.

TABLE 1 Comparative Comparative Transmittance Example 1 Example 2Example 3 Example 4 Example 1 Example 2 Base Oil PE Saturated Fatty AcidEster 65% 85.20 75.90 85.30 31.60 NPG Saturated Fatty Acid 70% 97.72Ester TMP Saturated Higher Fatty 65% 19.25 Acid Ester Copolymer ofUnsaturated 65% 10.55 10.45 Dibasic Acid Ester and α-Olefin TMPUnsaturated Fatty Acid 38% 66.20 Ester TMP Saturated/Unsaturated 45%70.33 Fatty Acid Ester TMP Saturated Lower Fatty 65% 27.35 Acid EsterAdditives Amine-Based Antioxidant (B1) 0.40 1.50 0.40 0.40 1.50Phenol-Based Antioxidant (B2) 1.00 0.50 1.00 1.00 1.40Sulfur-Phosphorus-Based Extreme-Pressure Agent 0.05 0.05 0.10 0.05 (C)PMA (1) 0.50 0.50 PMA (2) 2.00 2.00 2.00 Benzotriazole Compound 0.100.10 0.05 0.03 0.10 0.10 Rust Inhibitor (1) 0.10 0.10 0.20 0.10 0.10Rust Inhibitor (2) 0.10 Phosphorus-Based Extreme-Pressure Agent (1) 0.500.50 0.50 0.50 Phosphorus-Based Extreme-Pressure Agent (2) 0.02Anti-foam Agent 0.10 0.10 0.10 0.10 0.10 0.10 Total 100 100 100 100 100100 Transmittance of Total Amount of Composition 65% 65% 65% 70% 45% 48%Properties Kinematic Viscosity (40° C.) mm²/s 46.13 48.75 45.69 45.1345.43 48.80 Evaluation Kinematic Viscosity (100° C.) mm²/s 7.747 8.0107.770 7.977 8.765 8.818 Results of Viscosity Index 137 135 139 150 176162 Composition Acid Value (initial) mgKOH/g 0.28 0.18 0.36 0.16 0.910.61 Shell Wear mm 0.43 0.37 0.40 0.38 0.70 0.64 (50° C., 294 N, 30 min)Evaluation A A A A B B RBOT, 150° C., 620 kPa min 671 1035 694 390 67639 Evaluation A A A A B A ISOT (130° C. × 168 h) Viscosity Ratio (40°C.) — 1.02 1.02 1.00 1.00 3.29 1.26 Acid Value Increase mgKOH/g −0.020.09 0.16 0.05 14.69 0.96 Comparative Comparative ComparativeComparative Transmittance Example 3 Example 4 Example 5 Example 6 BaseOil PE Saturated Fatty Acid Ester 65% 31.55 81.50 NPG Saturated FattyAcid 70% Ester TMP Saturated Higher Fatty 65% Acid Ester Copolymer ofUnsaturated 65% 16.70 Dibasic Acid Ester and α-Olefin TMP UnsaturatedFatty Acid 38% 66.20 Ester TMP Saturated/Unsaturated 45% 69.78 71.13Fatty Acid Ester TMP Saturated Lower Fatty 65% 27.12 27.63 Acid EsterAdditives Amine-Based Antioxidant (B1) 1.50 1.20 0.35 0.40 Phenol-BasedAntioxidant (B2) 0.20 0.02 0.40 Sulfur-Phosphorus-Based Extreme-PressureAgent (C) PMA (1) 0.50 0.50 0.50 0.50 PMA (2) Benzotriazole Compound0.10 0.10 0.10 0.10 Rust Inhibitor (1) 0.10 0.10 0.10 0.10 RustInhibitor (2) Phosphorus-Based Extreme-Pressure Agent (1) 0.80 0.20Phosphorus-Based Extreme-Pressure Agent (2) 0.05 0.07 0.10 Anti-foamAgent 0.10 0.10 0.10 Total 100 100 100 100 Transmittance of Total Amountof Composition 48% 45% 48% 65% Properties Kinematic Viscosity (40° C.)mm²/s 48.44 45.34 47.65 46.88 Evaluation Kinematic Viscosity (100° C.)mm²/s 8.752 8.740 8.746 7.858 Results of Viscosity Index 161 176 165 137Composition Acid Value (initial) mgKOH/g 0.57 0.91 0.64 0.56 Shell Wearmm 0.66 0.65 0.62 0.63 (50° C., 294 N, 30 min) Evaluation B B B B RBOT,150° C., 620 kPa min 639 434 127 731 Evaluation A A B A ISOT (130° C. ×168 h) Viscosity Ratio (40° C.) — 1.25 1.40 3.56 1.01 Acid ValueIncrease mgKOH/g 0.93 2.17 7.18 −0.17 * The numeral values in thecolumns of base oil and additives are in terms of % by mass relative tothe total amount of the lubricating oil composition.

The components in Table 1 are as follows.

(Base Oil)

-   PE saturated fatty acid ester: complete ester of pentaerythritol and    a mixture of octanoic acid and decanoic acid (transmittance at    3,005±1 cm⁻¹: 65%)-   NPG saturated fatty acid ester: complete ester of neopentyl glycol    and isostearic acid (transmittance at 3,005±1 cm⁻¹: 70%)-   TMP saturated higher fatty acid ester: complete ester of    trimethylolpropane and isostearic acid (transmittance at 3,005±1    cm⁻¹: 65%)-   Copolymer of unsaturated dibasic ester and α-olefin: copolymer of    maleic acid butanol ester and α-olefin having 6 to 18 carbon atoms    (100° C. kinematic viscosity: 35 mm²/s, transmittance at 3,005±1    cm⁻¹: 65%)-   TMP unsaturated fatty acid ester: complete ester of    trimethylolpropane and oleic acid (transmittance at 3,005±1 cm⁻¹:    38%)-   TMP saturated/unsaturated fatty acid ester: complete ester of    trimethylolpropane and a mixture of isostearic acid and oleic acid    (transmittance at 3,005±1 cm⁻¹: 45%)-   TMP saturated lower fatty acid ester: complete ester of    trimethylolpropane and a mixture of caprylic acid and capric acid    (transmittance at 3,005±1 cm⁻¹: 65%)

(Additives)

-   Amine-based antioxidant (B1): 4-butyl-4′-octyldiphenylamine-   Phenol-based antioxidant (B2): n-octyl    3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate-   Sulfur-phosphorus-based extreme-pressure agent (C): compound    represented by the following chemical formula:

-   PMA (1): polymethacrylate (weight-average molecular weight: 180,000)-   PMA (2): polymethacrylate (weight-average molecular weight: 45,000)-   Benzotriazole compound: 1,2,3-benzotriazole-   Rust inhibitor (1): low-basic calcium sulfonate (total base number    28 mgKOH/g)-   Rust inhibitor (2): half ester of alkenylsuccinic acid and    polyalcohol-   Phosphorus-based extreme-pressure agent (1): tricresyl phosphate    (TCP)-   Phosphorus-based extreme-pressure agent (2): oleyl acid phosphate-   Anti-foam agent: silicone anti-foam agent

As described above, in Examples 1 to 4, the synthetic ester base oil (A)having a high transmittance was used in order that the transmittance at3,005±1 cm⁻¹ of the lubricating oil composition could be 50% or more,and the amine-based antioxidant (B1), the phenol-based antioxidant (B2)and the sulfur-phosphorus-based extreme-pressure agent (C) werecontained each in a predetermined amount, and therefore, the RBOT valueof the composition was sufficiently large and, in addition, theviscosity increase and the acid value increase in the ISOT test could beprevented from increasing, that is, the oxidation stability of thecomposition was good in various environments. Further, the wear loss inthe Shell wear test was small, and the wear resistance of thecomposition was good.

As opposed to these, in Comparative Examples 1 to 3, the transmittanceof the lubricating oil composition was less than 50%, and therefore inthe ISOT test, the viscosity increased and the acid value increased,that is, the oxidation stability of the composition could not besufficiently bettered. This tendency was also seen in ComparativeExamples 4 and 5 where both the amine-based antioxidant (B1) and thephenol-based antioxidant (B2) were contained. In Comparative Example 6,though the oxidation stability was good, the wear resistance could notbe enhanced since the composition did not contain the sulfur-phosphorusextreme-pressure agent (C).

1. A biodegradable lubricating oil composition, comprising (A) 0% bymass or more of a synthetic ester base oil (A); (B1) 0.1 to 3% by massof an amine-based antioxidant (B1); (B2) 0.1 to 3% by mass of aphenol-based antioxidant (B2); and (C) 0.01 to 2% by mass of asulfur-phosphorus-based extreme-pressure agent (C), wherein thetransmittance at 3,005±1 cm⁻¹ of a 0.1 mm-thick liquid film of thecomposition, as measured through IR absorptiometry, is 50% or more. 2.The biodegradable lubricating oil composition according to claim 1,wherein the synthetic ester base oil (A) comprises a polyol ester baseoil (A1) being an ester of a polyol and an aliphatic monocarboxylic acidin an amount of more than 50% by mass relative to the total amount ofthe synthetic ester base oil (A).
 3. The biodegradable lubricating oilcomposition according to claim 2, wherein the polyol ester base oil (A1)is an ester of a hindered polyol having one or more of quaternarycarbons in the molecule and having 1 to 4 methylol groups bonding to atleast one of the quaternary carbon atoms, and an aliphaticmonocarboxylic acid.
 4. The biodegradable lubricating oil compositionaccording to claim 1, wherein the synthetic ester base oil (A) comprisesat least a polyol ester base oil (A1-1) being an ester of a polyol andan aliphatic monocarboxylic acid and having a total carbon number of 23to 50 in one molecule in an amount of more than 50% by mass relative tothe total amount of the synthetic ester base oil (A).
 5. Thebiodegradable lubricating oil composition according to claim 4, whereinthe polyol ester base oil (A1-1) is at least one base oil selected fromthe group consisting of an ester of a neopentyl glycol and a saturatedaliphatic monocarboxylic acid having 9 to 22 carbon atoms, and an esterof a pentaerythritol and a saturated aliphatic monocarboxylic acidhaving 5 to 11 carbon atoms.
 6. The biodegradable lubricating oilcomposition according to claim 4, wherein the synthetic ester base oil(A) further comprises at least one base oil selected from the groupconsisting of a polyol ester base oil (A1-2) being an ester of a polyoland an aliphatic monocarboxylic acid and having a total carbon number of51 to 80 in one molecule and a copolymer (A3) of an unsaturated dibasicacid ester and an α-olefin in an amount of less than 50% by massrelative to the total amount of the synthetic ester base oil (A).
 7. Thebiodegradable lubricating oil composition according to claim 6, whereinthe polyol ester base oil (A1-2) is an ester of a trimethylolpropane anda saturated higher aliphatic monocarboxylic acid having 15 to 22 carbonatoms.
 8. The biodegradable lubricating oil composition according toclaim 1, wherein the sulfur-phosphorus-based extreme-pressure agent is adithiophosphate having a terminal carboxyl group.
 9. The biodegradablelubricating oil composition according to claim 1, wherein theamine-based antioxidant (B1) is a dialkyldiphenylamine.
 10. Thebiodegradable lubricating oil composition according to claim 1, whereinthe phenol-based antioxidant (B2) is an alkyl3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate.
 11. The biodegradablelubricating oil composition according to claim 1, further comprising:0.1 to 10% by mass of a viscosity index improver.
 12. The biodegradablelubricating oil composition according to claim 1, further comprising:0.01 to 1% by mass of a triazole compound.
 13. The biodegradablelubricating oil composition according to claim 1, which has abiodegradation rate of 60% or more as measured in a degradation test forchemical substances with microbes according to the 301B test of the OECDTest Guideline.
 14. The biodegradable lubricating oil compositionaccording to claim 1, which is for a hydraulic fluid.
 15. A method forproducing a biodegradable lubricating oil composition, the methodcomprising blending at least 0.1 to 3% by mass of an amine-basedantioxidant (B1), 0.1 to 3% by mass of a phenol-based antioxidant (B2)and 0.01 to 2% by mass of a sulfur-phosphorus-based extreme-pressureagent (C) in 50% by mass or more of a synthetic ester base oil (A), toproduce a biodegradable lubricating oil composition wherein thetransmittance at 3,005±1 cm⁻¹ of a 0.1 mm-thick liquid film of thecomposition, as measured through IR absorptiometry, is 50% or more.